Earthworms are segmented worms that are scientifically classified
as belonging to the phylum Annelida (ringed animal). There are 3500-4000
species of earthworms around the world. They are full of calcium,
protein, fibre and vitamins, making them a valuable food source
for many mammals, reptiles and fish. Earthworms vary in size, on
average from no more than 1 centimetre to about 3 metres in length.
One of the world’s largest earthworms, the Giant Gippsland
Earthworm (Megascolides australis), is found in Australia.
It has an average length exceeding 1 metre. However, the longest
recorded earthworm was a South African giant specimen (Microchaetus
rappi), measuring around 7 metres in length.
Historical renditions of the mighty earth movers
As they go about their daily business, largely obscured and forgotten
by the world above them, earthworms have been shown to consume and
have important effects on vast amounts of the soil beneath our feet.
It was just a few months after the British scientist Charles Darwin
returned from his famous voyage to the Galapagos Islands in 1837
that he went to visit his uncle in the country. As they walked in
the garden, the uncle pointed out a spot where he had spread ashes
and lime several years earlier. The spot had become buried in soil
cast by earthworms. Astonished to see that worms could move so much
soil, Darwin went home, lined the shelves of his study with glass-covered
pots full of earth and worms and began a series of experiments that
lasted 40 years. In one of the long experiments the scientist demonstrated
his patience. Darwin spread a layer of chalk on a grassy field and
waited 29 years to come back and dig a trench to see how deep the
earthworm castings piled up. He found the chalk was now sitting
below 6 inches (about 15 cm) of soil! In 1881 Darwin remarked “it
may be doubted whether there are many other animals which have played
so important a part in the history of the world, as have these lowly
organized creatures”.
Meanwhile in New Zealand in 1925 a farmer, Mr A S Ashmore, from
Raetihi became convinced that earthworms were not only moving large
amounts of soil around, but that their activities were having a
beneficial effect on the growth of the pasture on his farm. He was
so convinced by his observations that he began to deliberately introduce
earthworms to areas of his farm previously devoid of earthworms.
Later scientists were able to confirm Mr Ashmore’s observations
when they calculated that the introduction of earthworms had in
the long term improved the overall productivity of the pasture by
25-30%. Encouraging earthworm activities in the soil meant that
the farm was now able to carry 2.5 stock units per hectare more
than it could before – a clearly very beneficial effect. This
research sparked off a series of pioneering earthworm experiments
by a number of New Zealand scientists that ultimately have vastly
improved the current global understanding of earthworm ecology.
There are 3500-4000 species of earthworms in the world and nearly
200 species have been identified in New Zealand.
They are found in all but the driest and coldest parts of the
world, but generally prefer moist (but not too wet) environments.
Pastures commonly support the biggest populations of earthworms
as they usually contain large amounts of organic matter and are
infrequently disturbed by cultivation events.
Numbers of earthworms can commonly range between 7 and 12 million
per hectare under a productive pasture in New Zealand. This corresponds
to 1 to 3 tonnes of earthworms per hectare and means that the
weight of earthworms below a pasture is similar to the weight
of the grazing animals supported above ground!
Earthworms are hermaphroditic, which means they each have both
male and female organs.
It is an old wives’ tale that cutting an earthworm in half
will make two earthworms…. one part may survive, but it
is much more likely that both parts will die.
The excreta produced by earthworms is known as casts. As soil
passes through an earthworm’s body, some of the nutrients
are converted into forms that are more readily available for plant
uptake. So casts are generally rich in plant-available nutrients
like nitrogen, phosphorus, and potassium.
As well as consuming soil, earthworms also help to break down
and thereby recycle organic materials (such as dead herbage and
dung). Scientists have calculated that each year through their
activities they are responsible for burying around 6 tonnes of
pasture litter per hectare.
As they burrow through the soil, earthworms create burrows and
channels that help to loosen the soil, allowing air to circulate
and roots and water to penetrate the soil more easily.
In New Zealand an incredible 25-30 tonnes of soil per hectare
per year has been measured by scientists as being deposited by
earthworms on the soil surface in the form of casts. But we must
remember that some earthworms deposit their cast material both
within and upon the soil surface, so the total amount of soil
that they turn over in a year is even higher!
What are some of the benefits
of encouraging earthworms activity in soil?
improved physical structure of the soil
better drainage and aeration
enhanced soil fertility (nutrients become more readily available
to plants after they have been consumed by earthworms)
surface litter incorporation and recycling of nutrients back
into the soil
better water infiltration / reduced run-off of water
improved root penetration
Different earthworms have different
homes and
different jobs
It is important to realise that not all earthworms are the same – individual species have individual roles to play and functions
to carry out.
There are four main types of earthworms that you might find examples
of in your garden:
Compost dwellers. Like to live in high organic
matter environments such as compost heaps, but will not usually
survive in soil unless it contains very high amounts of organic
matter.
Soil surface dwellers. Feed on decaying roots,
shoots, leaves and dung and live near the soil surface (0-15 cm
depth). Important in mixing plant litter into the soil.
Topsoil dwellers. Most common earthworms in
New Zealand; live in the top 20-30 cm depth of soil. Burrow through
soil, eating and excreting it. Tend to eat more soil than organic
matter.
Subsoil dwellers. Tend to live in permanent
burrows up to as deep as 3 metres below soil surface. Drag food
such as leaves into their burrows from the soil surface. Often
larger than other earthworm types.
Examples of some earthworm species,
from www.crop.cri.nz
Compost preferring or soil surface dwelling
species
These species tend not to burrow into the soil. They prefer
to live at or near the soil surface or in compost heaps since
they like to eat material that is high in organic matter, e.g.
decaying plant roots and shoots, dung and leaves. Examples of
such species include the dung worm Lumbricus rubellus
and the tiger worm Eisenia fetida.
E. Fetida
L. rubellus
Topsoil dwelling species
These species usually live in the top 20-30 cm depth of soil.
They burrow through the soil, ingesting it as they go and thus
mix the topsoil layer (see diagram). As they burrow they produce
stable earthworm casts that help to improve soil structure and
increase soil aeration by creating cracks and channels in the
soil. The most common earthworm found in New Zealand is a topsoil
dwelling species called the grey worm, Aporrectodea caliginosa.
It is the most dominant species in our gardens.
A. caliginosa
O. cyaneum
Earthworm hole
The burrowing activity of earthworms creates burrows
and channels (worm holes) throughout the soil. These help to
loosen the soil, improving soil structure and fertility, and
create spaces for roots to grow through and air, water and fertiliser
to enter.
There are a number of tell-tale signs that indicate you may have
a lack of earthworms:
a turf mat of decomposing peat-like material may form near the
soil surface
old plant material remains on the soil surface and does not
break down
additions of fertiliser or lime to the soil surface do not get
mixed into the soil
How do I encourage earthworms in my garden?
Maintain soil pH between 5.8 and 6.3 by adding lime periodically
to the soil. Limit the amount of cultivation where possible but,
if cultivating, avoid machines that pulverise the soil and the
earthworms contained therein. Limit the use of harmful pesticides
(particularly fungicides and fumigants).
Irrigate the soil during dry periods to maintain earthworm activity,
and increase organic matter in the soil (a good food source for
earthworms) by incorporating composted material or animal manure.
